Blog by Dr Malcolm Burgess, Senior Conservation Scientist, RSPB Centre for Conservation Science

At the RSPB Centre for Conservation Science we have been trying to diagnose the causes of wood warbler decline since 2009, initially focusing on potential problems that may affect them during the breeding season. Working in Wales, we collected data to compare with similar collected in the 1980s before the onset of their nationwide population decline. We found that nest success rates had not changed, and that changes in invertebrate food availability and woodland habitat quality were not related to either breeding success or population change. We also established that wood warblers are not negatively affected by earlier springs, when the peak abundance of caterpillars (preferred wood warbler food when available) occurs too early to be available to nestlings. You can read more about all this research here.

This work suggested that the problems facing wood warblers may be occurring elsewhere along the flyway, either along their migration routes and/or on their African non-breeding grounds. However, as with many other long distance migrants, a significant problem we have is a very poor knowledge of where these areas even are. Ringing recoveries indicate wood warblers stopover in Italy in late summer, but no UK ringed wood warblers have ever been recovered in sub-Saharan Africa.

Pan-European work using stable isotopes suggested most of Europe's breeding wood warblers, including UK birds, spend the winter in the humid forest zone of the Congo basin in central Africa. However the precision and reliability of this technique for identifying non-breeding ranges in sub-Saharan Africa is untested. Plus we know wood warbler distribution in Africa extends much further west than this work suggests. We have undertaken work in Ghana, where we found that wood warblers are species of well-wooded farmland, rather than of pristine forest, as may have been expected. We have also found stopover sites in Burkina Faso, used by birds after having crossed the Sahara, and have discovered that individual birds return to both stop-over and wintering sites in subsequent years.

Photo: Wood warbler with tag, by Joan Castello

A continuing aim of our work in the UK is to examine survival through uniquely colour marking adults and young. Each year we find and monitor about 40 nests, thus capturing a high proportion of the adults breeding in our Dartmoor study population and marking many young. Individual wood warblers typically show low fidelity to breeding areas between years, though in our study population it is relatively high for the species: on average 17% of males return the following year and we find high levels of young recruitment (about 8% return to breed on Dartmoor).

Relating the inter-annual differences in survival to events occurring outside the UK, such as weather conditions, requires decades of data and knowledge of the routes taken and areas used. Up to now, the low weight of the birds has prevented us from taking advantage of tracking technology to answer some of these questions. However, since starting our work on wood warblers, miniaturised tracking devices have been getting ever smaller and lighter.

Tagging birds to research their migration

Wood warblers only weigh around 10g and so any tag would have to weigh less than 0.4g for us to contemplate tagging. Last year such a tag was made available, a geolocator weighting just 0.36g including the harness to attach it safely to the birds back. Geolocators record light levels every few minutes, enabling us to estimate the time of sunrise and sunset each day and from this estimate the location of the bird. Using these devices we hope to establish where UK breeding wood warblers go between breeding seasons, the routes they take and the timings of their migrations. Knowing this will enable us to look at potential impacts of events across their annual cycle on population trends. In partnership with the British Trust for Ornithology (BTO) and Devon Birds, in 2016 we together fitted 27 geolocators to adult male wood warblers, 17 on Dartmoor and 10 in the New Forest.

Fast forward a year to this spring, and we eagerly and nervously await the return of male wood warblers to our Dartmoor woods. Most of the early arrivals were unringed, but soon we found two of the 53 uniquely colour marked untagged birds we marked in 2016 to be able to compare return rates with tagged birds. And then we found one of our tagged males!

We returned a couple of days later with a film crew filming our work for a new BBC2 series 'Hugh's Wild West' to catch him and retrieve the tag - we have to do this to download the data as these small tags do not transmit. Usually males sing strongly and are relatively easy to locate and capture, so we were disappointed at the silence when we arrived on his territory. We hoped a sparrowhawk we flushed exactly where the male had been singing previously explained the lack of singing, but unfortunately the male never sang and after a couple of hours we gave up on him. We did find the plucked remains of a blackbird which at least took away the fear of a sparrowhawk predation!

Photo: Wood warbler perched on branch, by Andy Hay (rspb-images.com)

The film crew and I left colleague Joan Castello to continue searching for males further up the valley for the remainder of the afternoon. Late in the afternoon Joan texts to say he had found a second loggered male singing! The film crew were unable to return for several days so we decided not to take the risk of losing this male and returned early the next day. We captured him and recovered the tag. Meanwhile, a few weeks later in the New Forest one of the BTO’s 10 tagged males was found, captured and his tag also retrieved.

We eventually re-found the missing Dartmoor tagged male, not far from where he was first seen. The film crew returned, but frustratingly we were unable to catch him. That’s wildlife for you, never reliable for TV appearances! This male was unpaired and carried on singing for a few days but once again disappeared when the cameras tried again to film his capture. We then found a nest in the same area which gave us hope, but no male was in attendance during incubation and sadly the nest was predated before the chick stage, so we were never sure if it was his nest. We didn't find this male again so never retrieved his tag - but at least we have one retrieved from Dartmoor and one from the New Forest. The data from both these tags are currently being analysed and we should have some exciting results to share in the autumn... We’ll keep you updated on our findings!

We’d like to thank Devon Birds, the BTO, and Natural England for supporting this work.

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